Morphology engineering of conductive metallic nanoparticles capped with an organic coating

ABSTRACT

The present invention is directed to a sensor having continuous and discontinuous regions of conductive metallic nanoparticles capped with an organic coating which enables the detection of volatile organic compounds and/or water vapor.

FIELD OF THE INVENTION

The present invention is directed to the detection of volatile organiccompounds and water vapor using a sensor comprising continuous anddiscontinuous regions of conductive metallic nanoparticles capped withan organic coating, wherein the continuous and discontinuous regionsdifferentially detect volatile organic compounds and water vapor.

BACKGROUND OF THE INVENTION

Monolayer-capped metallic nanoparticles (MCNPs) have attractedsignificant attention during the past two decades, due to their uniquebulk and surface properties (Mokari, Nano Rev. 2011, 2, 5983; Tisch &Haick, MRS Bull. 2010, 35(1), 797; Tisch & Haick, Rev. Chem. Eng. 2010,26, 171; Haick, J. Phys. D 2007, 40, 7173; Daniel & Astruc. Chem. Rev.2004, 104, 293). Sensors based on MCNP films are of special interest,mainly due to their controllable selectivity, high sensitivity, lowdetection limits, fast response and recovery times, small size,low-output impedance and easy integration in standard microelectronicdevices (Joseph et al., J. Phys. Chem. C 2008, 112, 12507; Joseph etal., Sens. Actuat. B 2004, 98, 188; Herrmann et al., Phys. Rev. B 2007,76, 212201; Wang et al., Langmuir 2010, 26, 618; Dovgolevsky et al.,Small 2009, 5, 1158). These features allowed the successfulimplementation of MCNPs in a wide variety of applications, varying fromdetection of organic compounds in aqueous solutions (Raguse et al., J.Phys. Chem. C. 2009, 113, 15390; Chow et al., Sens. Actual. B 2009, 143,704; Cooper et al., J. Anal. Chem. 2010, 82, 3788) to detection of traceanalytes in the gas phase (Joseph et at, Sens. Actuat. B 2004, 98, 188;Joseph et al., J. Phys. Chem. B 2003, 107, 7406; Dovgolevsky et al., J.Phys. Chem. C 2010, 114, 14042), and even diagnosis of diseases frombreath samples (Tisch & Haick, Rev. Chem. Eng. 2010, 26, 171; Peng etal., Nature Nanotechnol. 2009, 4, 669; Peng et al., Br. J. Cancer 2010,103, 542; Barash et at, Small 2009, 5, 2618; Hakim et al., Br. J. Cancer2011, 104, 1649; Marom et al., Nanomed. (Future Medicine) 2012, 7, 639;Shuster et al., Breast. Cancer Res. Treat. 2011, 126, 791) or in vitrosamples (Barash et al., Small 2009, 5, 2618; Barash et al., Nanomed,Nanotech. Bio. Med. 2012, 8, 580). For use in chemiresistors, the metalcores, consisting either of a single metal or of an alloy of two or moremetals, provide the electronic conductivity. The non-conductive organicmatrix that coats the nanoparticles provides adsorption sites for theanalyte molecules. The combination of metal cores and capping organicmatrix provides two counteracting effects upon analyte adsorption: (i)three-dimensional swelling of the MNCP film that increases theinterparticle tunneling distance for charge carriers and, hence, thefilm's resistance; and (ii) increasing the permittivity of the organicmatrix around the metal cores that decreases the potential barriersbetween the metal cores, and, hence, the film's resistance (Haick, J.Phys. D 2007, 40, 7173). To tune the sensing signals of the MCNPs, apopular approach has been applied, namely to use derivatives ofdifferent backbones (Wang et al., J. Mater. Chem. 2007, 17, 457; Rowe etal., Chem. Mater. 2004, 16, 3513) and/or different electron-withdrawingor electron-accepting functional groups (Cooper et al., J. Anal. Chem.2010, 82, 3788; Joseph et al., J. Phys. Chem. C. 2007, 111, 12855). Thisapproach, however, incorporates synthesis challenges either of themolecular ligands or the MCNPs per se. In addition, different ligandsusually result in different steric hindrance between the adjacentligands adsorbed on the NP surface, different molecular densities on theNP surface, and therefore, different NP sizes and/or NP sizedistribution (Wang et al., J. Mater, Chem. 2007, 17, 457; Rowe et al.,Chem. Mater. 2004, 16, 3513; Murphy et al., J. Phys. Chem. B 2005, 109,13857). These changes affect the signal features, the chemicalselectivity, the morphology, the baseline resistance, and/or thestability and performance over time of the MCNP chemiresistive films(Wang, et al., J. Mater. Chem. 2007, 17, 457; Garg et al.,Nanotechnology 2010, 21, 405501; Wang et al., Langmuir 2010, 26, 618;Nath & Chilkoti, in Engineering in Medicine and Biology, 2002. 24^(th)Annual Conference and the Annual Fall Meeting of the BiomedicalEngineering Society EMBS/BMES Conference, 2002, Proceedings of theSecond Joint, Vol. 1, 2002, pp. 574-575; Rowe et al., Chem. Mater. 2004,16, 3513; Joseph et al., J. Phys. Chem. C. 2007, 111, 12855; Cooper etal., J. Anal. Chem. 2010, 82, 3788; Joseph et al._(;) J. Phys. Chem. B2003, 107, 7406). As an illustrative example, Han et. al. (Anal. Chem.2001, 73, 4441) have shown that a change in the NP size from 5 nm to 2nm could affect the sensitivity of a specific MCNP chemistry, undersimilar exposure conditions to analytes, by ca. 35%. Dasog et al.(Langmuir 2007, 23, 3381) have shown that changes in the core sizeaffect the oxidation rate of the organic ligands adsorbed on the NPsurface, causing different drift in the sensing signal.

Joseph et al. (J. Phys. Chem. C 21108, 112, 12507) showed that, forindividual MCNP-dominated morphology, the MCNP film is not conductiveand the chemiresistor device shows no response. For island-dominatedmorphology, charge transport becomes possible after a ID percolationpathway is formed. This percolation pathway contains largeisland-to-island gaps, which are the bottlenecks for charge transportdue to their high resistance. Changes in permittivity (Haick, J. Phys. D2007, 40, 7173; Joseph et al., J. Phys. Chem. B 2003, 107, 7406;Steinecker et al., Anal. Chem. 2007, 79, 4977) and swelling-inducedreduction in the island-to-island gaps decrease the resistance of MCNPfilms. For continuous 3D morphology, where many percolation pathwaysexist, the MCNP film swells along the direction perpendicular to thesurface after dosage with analytes. As a result, the interparticledistances along these percolation pathways increase, and accordingly,the resistance of the MCNP film increases. Likewise, a decrease(shrinking) in the interparticle separation within the continuous 3Dmorphology usually leads to a decrease in resistance. The effect of thedielectric constant of the analyte on the sensing signal is presumablyweaker, thus reducing the magnitude of the positive response for some ofthe analytes. This explanation is based on the assumption that thesensing mechanism remains the same (swelling/shrinking and dielectricpermittivity changes) when percolation pathways have already beenformed. Despite advances in this field, many of the underlying molecularmechanisms generating the sensing signal remain only vaguely understood(Haick, J. Phys. D 2007, 40, 7173; Kane et al., J. Mater. Chem. 2011,21, 16846; Zabet-Khosousi & Dhirani, Chem. Rev. 2008, 108, 4072; Shusteret al., J. Phys. Chem. Lett. 2011, 2, 1912).

WO 2009/066293, WO 2009/ 118739, WO 2010/079490, WO 2011/148371, WO2012/023138, US 2012/0245434, and US 2012/0245854 to some of theinventors of the present invention disclose apparatuses based onnanoparticle conductive cores capped with an organic coating fordetecting volatile and non-volatile compounds, particularly for cancerdiagnosis.

A problem often encountered in the diagnosis of diseases through theanalysis of volatile organic compounds (VOCs) in breath samples is thesensitivity of sensing apparatuses to humidity. Since breath samples maycontain up to 80% relative humidity (RH), the VOCs are often masked bywater vapor which consequently impedes sensor performance.

Han et al. (Chem. Phys. Lett. 2002, 355, 405) studied the effects ofrelative humidity on the conductance of the assembly ofpoly(dG)-poly(dC) and poly(dA)-poly(dT) DNA molecules. The results showthat the conductance of a specimen consisting of multiple DNA moleculesmight be strongly affected by the relative humidity. A similar effectwas reported for silica gel surfaces (Anderson & Parks, J. Phys. Chem.1968, 72, 3662). Guo et al., (Guo et al., Sens. Actuat. B 2007, 120,521) showed that when alkanethiol-capped AuNP films that containedtraces of the phase-transfer reagent tetraoctylammonium bromide (TOABr)were used as chemiresistive sensors, the film resistance decreased whenthe sensors were exposed to water vapor.

In order to overcome the effect of water vapor on sensor performance, asensing apparatus is typically equipped with a humidity sensor (Yell &Tseng, J. Mat. Sci. 1989, 24, 2739) that independently measures thecontent of water vapor to be subtracted from the sensing signal thusaffording the extraction of VOCs' signal.

There remains an unmet need of a sensing apparatus for detectingmixtures of VOCs in breath samples without dehumidifying the sampleprior to measurement. There further remains a need for a humidity sensorhaving fast and reversible response upon exposure to water vapor.

SUMMARY OF THE INVENTION

The present invention provides a method of controlling the sensitivityof monolayer-capped metallic nanoparticle (MCNP) chemiresistive sensorstowards volatile organic compounds (VOCs) and/or water vapor by varyingthe morphology of MCNP films to comprise continuous and discontinuousregions thereby enabling the differential detection of water vapor andVOCs. The present invention further provides concurrent detection ofvolatile organic compounds (VOCs) and water vapor using a single sensorand a process of manufacturing said sensor.

The present invention is based in part on the unexpected finding that asensor comprising a continuous film of MCNP provides positive responsesupon exposure to VOCs and to water vapor while a sensor comprising afilm of MCNP which has discontinuous regions provides positive responsesupon exposure to VOCs, but negative responses upon exposure to watervapor. The negative responses were at least 1 order of magnitude largerthan the positive responses. Thus, by engineering the morphology of MCNPfilms mainly by systematic control of the percentage of film coverage ofa substrate, concurrent detection of VOCs and water vapor can beachieved. These results provide a new avenue to tailor the sensingproperties of the MCNP chemiresistors, thus widening the spectrum ofpotential applications of these sensors from VOCs' breath analysis tohumidity detection.

According to a first aspect, the present invention provides a sensor fordetecting an analyte selected from a volatile organic compound, watervapor and combinations thereof, the sensor comprising continuous anddiscontinuous regions of conductive metallic nanoparticles capped withan organic coating, wherein the continuous and discontinuous regionsdifferentially detect water vapor and volatile organic compounds.

According to one embodiment, the sensor provides the concurrentdetection of volatile organic compounds (VOCs) and water vapor.

According to another embodiment, the present invention provides a sensorarray comprising a plurality of sensors, each sensor comprisingcontinuous and discontinuous regions of conductive metallicnanoparticles capped with an organic coating, wherein the continuous anddiscontinuous regions differentially detect water vapor and VOCs. Insome embodiments, the sensor array comprises between 2 and 20 sensors,for example 2, 4, 6, 8, 10, 12, 14, 16, 18 or 20 sensors. Eachpossibility represents a separate embodiment of the present invention.

In other embodiments, the sensor comprises a of conductive metallicnanoparticies capped with an organic coating, the film comprisingcontinuous and discontinuous regions. In further embodiments, the filmcomprising continuous and discontinuous regions of conductive metallicnanoparticies capped with an organic coating is formed on a substrate.In additional embodiments, the film comprising continuous anddiscontinuous regions of conductive metallic nanoparticles capped withan organic coating is characterized by a thickness ranging from aboutIran to about 500 nm.

In some embodiments, the sensor is configured in a form selected fromthe group consisting of a capacitive sensor, a resistive sensor, achemiresistive sensor, an impedance sensor, and a field effecttransistor sensor. Each possibility represents a separate embodiment ofthe present invention.

In additional embodiments, the sensor is a chemiresistor comprising afilm comprising continuous and discontinuous regions of conductivemetallic nanoparticies capped with an organic coating formed on asubstrate.

In particular embodiments, the substrate is a rigid substrate or aflexible substrate. Each possibility represents a separate embodiment ofthe present invention. In some embodiments, the substrate is selectedfrom the group consisting of metals, insulators, semiconductors,semimetals, polymers, and combinations thereof Each possibilityrepresents a separate embodiment of the present invention. In otherembodiments, the substrate is a polymer selected from the groupconsisting of polyimide, polyatnide, polyimine, polyester,polydimethylsiloxane, polyvinyl chloride, and polystyrene. Eachpossibility represents a separate embodiment of the present invention.In one embodiment, the substrate comprises silicon dioxide. In anotherembodiment, the substrate comprises indium tin oxide.

In further embodiments, the continuous regions exhibit a positiveresponse upon exposure to VOCs and to water vapor, and the discontinuousregions exhibit a positive response upon exposure to VOCs and a negativeresponse upon exposure to water vapor.

In other embodiments, the discontinuous regions comprise voids rangingin size from about 10 nm to about 500 nm.

In various embodiments, the discontinuous regions comprise between about3% and about 90% of voids.

In certain embodiments, the conductive metallic nanoparticles areselected from the group consisting of Au, Ag, Ni, Co, Pt, Pd, Cu, and Alnanoparticles and combinations thereof Each possibility represents aseparate embodiment of the present invention. In an exemplaryembodiment, the conductive metallic nanoparticles are gold (Au)nanoparticles

In other embodiments, the conductive metallic nanoparticles have ageometry selected from the group consisting of a cubic, a spherical, anda spheroidal geometry. Each possibility represents a separate embodimentof the present invention.

In various embodiments, the organic coating forms a monolayer on top ofthe conductive metallic nanoparticles. In specific embodiments, theorganic coating comprises compounds selected from the group consistingof alkylthiols, arylthiols, alkylarylthiols, alkylthiolates,ω-functionalized alkanethiolates, arenethiolates,(γ-mercaptopropyl)tri-methyloxysilane, dialkyl disulfides andcombinations and derivatives thereof. Each possibility represents aseparate embodiment of the present invention. In further embodiments,the organic coating is 2-nitro-4-trifluoro-methylbenzenethiol. In otherembodiments, the organic coating is 3-ethoxythiophenol.

In additional embodiments, the present invention provides a method fordetecting an analyte selected from a volatile organic compound, watervapor and combinations thereof in the breath of a subject or in asample, the method comprising the steps of (i) providing a sensorcomprising continuous and discontinuous regions of conductive metallicnanoparticles capped with an organic coating, wherein the continuous anddiscontinuous regions differentially detect water vapor and VOCs; (ii)exposing the sensor to the breath of a subject or to a sample comprisingsaid analyte; and (iii) detecting a signal generated by said analyteusing a detection means.

In further embodiments, the method of the present invention provides thedetection of volatile organic compounds (VOCs), while concurrentlydetermining the amount of water vapor, the method further comprising thesteps of: (iv) determining the amount of water vapor from the detectedsignal; and (v) subtracting the amount of water vapor from the detectedsignal thereby allowing the detection of VOCs in the breath of a subjector in said sample. In accordance with these embodiments, the step ofdetecting a signal generated by said analyte using a detection meanscomprises the detection of a signal generated by VOCs and water vaporusing different applied voltages.

According to a second aspect, the present invention provides a systemfor detecting an analyte selected from a volatile organic compound,water vapor and combinations thereof, the system comprising: (i) asensor array comprising a plurality of sensors, each sensor comprisingcontinuous and discontinuous regions of conductive metallicnanoparticles capped with an organic coating, wherein the continuous anddiscontinuous regions differentially detect water vapor and VOCs; and(ii) a pattern recognition analyzer, wherein the pattern recognitionanalyzer receives sensor output signals and compares them to storeddata.

In certain embodiments, the pattern recognition analyzer comprises atleast one pattern recognition algorithm. Suitable pattern recognitionalgorithms include, but are not limited to, artificial neural networks,multi-layer perception (MLP), generalized regression neural network(GRNN), fuzzy inference systems (FIS), self-organizing map (SOM), radialbias function (RBF), genetic algorithms (GAS), neuro-fuzzy systems(NFS), adaptive resonance theory (ART) and statistical methodsincluding, but not limited to, principal component analysis (PCA),partial least squares (PLS), multiple linear regression (MLR), principalcomponent regression (PCR), discriminant function analysis (DFA)including linear discriminant analysis (LDA), and cluster analysisincluding nearest neighbor. Each possibility represents a separateembodiment of the present invention.

In some embodiments, the method for detecting volatile organic compounds(VOCs), while concurrently determining the amount of water vaporcomprises the use of a system comprising a plurality of sensors, eachsensor comprising continuous and discontinuous regions of conductivemetallic nanoparticles capped with an organic coating, and a patternrecognition analyzer, the method further comprises the step of analyzingthe detected signal using a pattern recognition analyzer which receivessensor output signals and compares them to stored data.

In various embodiments, the detection means comprises a device formeasuring changes in resistance, conductance, alternating current (AC),frequency, capacitance, impedance, inductance, mobility, electricalpotential, optical property or voltage threshold. Each possibilityrepresents a separate embodiment of the present invention.

In some embodiments, the VOCs to be detected are selected from polarorganic molecules, non-polar organic molecules and combinations thereof.Each possibility represents a separate embodiment of the presentinvention.

According to yet another aspect, the present invention provides a methodof manufacturing a sensor for detecting an analyte selected from avolatile organic compound, water vapor and combinations thereof, themethod comprising the step of forming a film comprising continuous anddiscontinuous regions of conductive metallic nanoparticles capped withan organic coating on a substrate, wherein the discontinuous regionscomprise voids which are produced in the presence of water vapor duringfilm formation process. In accordance with these embodiments, the watervapor present during film formation process is used to control the sizesand percentages of the voids in said discontinuous regions.

In certain embodiments, the amount of water vapor present during filmformation process ranges from about 1% to about 20% relative humidity.In particular embodiments, the amount of water vapor present during filmformation process ranges from about 5% to about 10% relative humidity.

In further embodiments, the sensor, sensor array and system of thepresent invention can be used for breath analysis, wherein the analyteto be detected is a VOC or mixtures thereof.

In exemplary embodiments, the present invention provides the diagnosisof various diseases in a subject, preferably a human, through thedetection of VOCs indicative of said diseases. Encompassed within thescope of the present invention is the diagnosis of cancer through thedetection of breath VOCs which are indicative of cancer. In someembodiments, the present invention provides the diagnosis of cancerselected from the group consisting of lung, brain, ovarian, colon,prostate, kidney, bladder, breast, oral, and skin cancers. Eachpossibility represents a separate embodiment of the present invention.

In additional embodiments, the sensor and sensor array of the presentinvention can be used as a means to detect relative humidity. In oneembodiment, the present invention provides a breath sensor, wherein saidbreath sensor detects the presence of proximate breath through thedetection of its relative humidity. In accordance with theseembodiments, the breath sensor can be used for monitoring breath thusaffording the diagnosis and treatment of sleep apnea and otherrespiratory diseases, and the prevention of sudden infant death syndrome(SIDS). The present invention thus provides a method of monitoring thebreathing of a subject comprising detecting a signal generated by watervapor present in each breath of a subject. The sensor/sensor array ofthe present invention may be in a form of a nose clip or a stick-patch,optionally connected to a device which produces a human perceptiblesignal (e.g. an audible alarm) when breath is not being detected.

In further embodiments, the present invention provides a method ofactivating an input device by a subject comprising detecting a signalgenerated by water vapor present in the breath of said subject, andchanging the configuration of a switch by the detected signal therebyactivating said input device. In some embodiments, the sensor of thepresent invention operates as a humidity sensor which activates an inputdevice by an on/off switch (e.g. humidity-triggered switch). Inaccordance with these embodiments, the detection of a signal generatedby water vapor present in exhaled breath of a subject is being used tochange the configuration of a switch thus enabling the activation of aninput device. In certain embodiments, the sensor of the presentinvention can be used to turn on a switch by the detection of watervapor from breath directly exhaled by the subject, wherein the subjectis selected from the group consisting of a quadriplegic patient, aparaplegic patient, an amputee, a patient with a spinal cord injury, aphysically impaired person, and any combination thereof. Eachpossibility represents a separate embodiment of the present invention.In further embodiments, the input device comprises at least one of a“Sip and Puff” switch, a pager button, an emergency button, an on/offbutton for universal remote devices (e.g. TV, AC, Video, Stereo, and thelike), a light switch, a door opening switch optionally combined with avideo-intercom, an elevator button, a smartphone controller, and aselection (click) button for an IR guided cursor. Each possibilityrepresents a separate embodiment of the present invention.

Further embodiments and the full scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. However, it should be understood that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F. FE-HRSEM images of three sensors produced by drop castingNTMBT-AuNP solution on the electrode structure at three differentrelative humidity (RH) conditions as follows: (FIGS. 1A, 1B) 16% RH(Si); (FIGS. 1C, 1D) 31% RH (S2); and (FIGS. 1E, 1F) 54% RH (S3).

FIG. 2. HR-SEM images of representative S1 (columns i and ii) and S3(columns iii and iv) films at different magnifications (panels A-D),using both BSE (columns i and iii) and SE (columns ii and iv) detectors.

FIGS. 3A-3B. AFM images of: (FIG. 3A) S1 and (FIG. 3B) S3 films.

FIGS. 4A-4C. Normalized responses ΔR/R_(end), of three randomlyselected: (FIG. 4A) S1 sensors, (FIG. 4B) S2 sensors, and (FIG. 4C) S3sensors, produced in three different casting cycles (cycle A, black;cycle B, red; and cycle C, blue), upon exposure to different levels ofRH.

FIGS. 5A-5D. Normalized responses ΔR/R_(end), of S1 (black), S2, (red)and S3 (blue) to various concentrations of: (FIG. 5A) 2-ethylhexanol;(FIG. 5B) decane; and (FIG. 5C) water vapor. (FIG. 5D) ΔR/R_(end) of S3upon exposure to various RHs. R_(end) is the resistance at the end ofthe sensing signal and ΔR is the R_(end)-corrected resistance changeupon exposure of the sensor to the analyte.

FIGS. 6A-6B, Normalized responses, ΔR/R_(base) of: (FIG. 6A) NTMBT-AuNPsensors (S4, black; S5, red; and S6, green); and (FIG. 6B) ETP-AuNPsensors (S7, black; S8, red; and S9, blue) on flexible substrate vs.humidity in a constant temperature. R_(base) is the baseline resistance.

FIGS. 7A-7C. Normalized responses, ΔR/R_(base) of: ETP-AuNP sensorsversus humidity with 0.2 seconds response time (blue) and 1 minuteresponse time (red). (FIG. 7A) ETP-AuNP sensor having baselineresistance of 18 MΩ (S7); (FIG. 7B) ETP-AuNP sensor having baselineresistance of 29 MΩ (S8); and (FIG. 7C) ETP-AuNP sensor having baselineresistance of 31 MΩ (S9).

FIGS. 8A-8D. Normalized current-voltage (I-V) curves for (FIG. 8A) S1,(FIG. 8B) S2, and (FIG. 8C) S3 at ˜20% RH (black), 40% RH (red) and 50%RH (blue). (FIG. 8D) Magnification of the −0.005V to +0.005V region ofthe S3 sensor, presented in FIG. 8C.

FIGS. 9A-9C. Bromothymol Blue indicator on an interdigitated electrodeafter drying (FIG. 9A) without applied voltage; (FIG. 9B) with appliedvoltage (5 V). (FIG. 6C) Magnification of the central area of FIG. 9B.

FIG. 10. Calculated relative response of S2 to 40% RH compared to 5% RR,under different voltages (−5.0 V black; −3.0 V, red; −1.5 V, blue; −1.0V, pink; and −0.9 V, green).

FIG. 11. A schematic representation of the response of monolayer-cappedmetallic nanoparticle (MCNP) chemiresistors on a SiO₂ substrate tovolatile organic compounds (VOCs) and water vapor.

FIG. 12. Resistance vs. time of NTMBT-AuNPs in a single breath cycle.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for determining the presence ofan analyte selected from a VOC, water vapor and combinations thereof ina sample. The present invention thus provides a sensor which comprisesconductive metallic nanoparticles capped with an organic coating thatare arranged in continuous and discontinuous regions which allow thedifferential detection of VOCs and water vapor.

Water molecules are a main confounding environmental factor that screensthe sensing signals obtained from VOCs in breath analysis applications.Unexpectedly it is now disclosed that by engineering the morphologyproperties of MCNP films via controlling their coverage on the devicesurface, double information can be extracted from the breath sample, bymeasuring both the amount of water vapor and the presence of breathbiomarker VOCs using the same sensor. The present inventors havediscovered unexpectedly, that continuous MCNP film morphology exhibitspositive responses upon exposure to VOCs and to water vapor, consistentwith the swelling and dielectric sensing mechanisms. In contrast,discontinuous films exhibit positive responses upon exposure to VOCs,but a high-magnitude negative response upon exposure to water vapor.These findings are attributed to ionic conduction from the MCNP freedomains (voids on the surface) to the MCNP domains, where conductionthrough tunneling also exists.

Nanoparticle (NP) based chemiresistors typically exhibit two types ofresponses: a reduction or enhancement of resistivity. While smallresponse values, up to 10-15%, could be explained by the swelling andthe dielectric phenomenon when conduction is by tunneling betweenadjacent NPs, some sensors exhibit a very large relative response of upto -90% and more, which cannot be explained by this model. Asdemonstrated herein, the negative responses result fromspecific-morphology of NPs based sensors and are correlated with thepresence and size of a hysteresis loop in the I-V scan. This largenegative response (as well as the hysteresis loop) is shown to be theresult of ionic conduction through condensate water over the substrate.This happens when the NPs' film is perforated, namely it comprisesvoids. Water vapor condensate on these voids and the applied voltageionizes the water and/or the silicone dioxide (the substrate), allowingcurrent to flow. This conduction does not involve the VOCs. Hence thiseffect is only proportional to the amount of condensed water. Theionization is dependent on the applied voltage. Thus, at differentvoltages there are more or less conductive ionic species (for a givenamount of water condensate on the surface). By probing at differentvoltages, it is possible to extract the contribution of ionic conductionfrom the overall conduction by tunneling and ionic flows together. Asdemonstrated herein, higher humidity levels provide higher conduction ofionic species using the same applied voltage. Since the ionic conductionis proportional to the humidity level (the amount of condensed water onthe surface), it is possible to calculate the relative humidity of thesurrounding from the ionic current. Preliminary calibration by exposuresto known amounts of water vapor or known relative humidity levels andmeasuring the thus obtained responses can also be performed.

The present invention overcomes the drawbacks of the background art byproviding the use of a single sensor for detecting volatile organiccompounds and water vapor. US 2010/0191474 and Dovgolevsky et al. (J.Phys. Chem. C 2010, 114, 14042) to one of the inventors of the presentinvention disclose sensors which exhibit low sensitivity to polaranalyte vapor, especially water vapor. It was not previously realizedthat a film of nanoparticles which comprises continuous anddiscontinuous regions can be utilized as a sensor which is sensitive toboth non-polar and polar VOCs including water vapor, thereby enablingthe concurrent detection of VOCs and water vapor in a sample. For breathanalysis applications, the reduction of water from the signal of VOCs ofinterest is crucial since water is not a marker of interest, but ispresent in breath samples in large amount (as high as 80% relativehumidity). The present invention provides a novel method to achieve thisgoal by controlling the morphology of MCNP, without the need forcomplicated synthesis approaches of the organic ligands and/or MCNPs.The effect of morphology variations within a continuous MCNP film, wherepercolation pathways already exist, on the sensing of volatile organiccompounds (VOCs) and water molecules enables to widen the spectrum ofpotential applications of MCNP chemiresistors.

Currently available humidity sensors include capacitive humiditysensors, resistive humidity sensors, dew point and thermal conductivity.For most applications, the environmental conditions dictate the choiceof sensor technology. Resistive, capacitive, and thermal conductivitysensing technologies each offer distinct features. Resistive sensors areinterchangeable, usable for remote locations, and cost effective.Capacitive sensors provide wide RH range and condensation tolerance,and, if laser trimmed, are also interchangeable. Thermal conductivitysensors are suitable for corrosive environments and high temperatures.All of these sensors provide response times which range front severalseconds to tens of seconds, with the time required to return to baselinebeing even longer. Most dew point based sensors are large and relativelyexpensive. The average response time of most sensors remains above 1second. Thus, these sensors are often not sensitive to rapid changes inhumidity on the scale of single seconds.

The sensor of the present invention provides rapid responses to smallchanges in humidity with a response time of less than I second. Thepresent invention thus offers a humidity sensor having relatively smalldimensions and which produces fast response and return (recovery) timeswith low production costs and power consumption. The present inventionprovides a sensor for the detection of a VOC or water vapor or theconcurrent detection of both for use as a breath analyzer, a breathsensor or a breath switch having very fast response and return times (<1second) and high sensitivity to VOCs as well as water vapor (<1% changesin RH. are detectable).

Disclosed herein is a sensor comprising metallic nanoparticles cappedwith an organic coating, wherein the metallic nanoparticles capped withan organic coating are arranged in continuous and discontinuous regionsthus allowing the differential detection of VOCs and water vapor.According to the principles of the present invention, continuous MCNPfilm morphology exhibits positive responses upon exposure to VOCs and towater vapor while discontinuous MCNP film morphology exhibits positiveresponses upon exposure to VOCs and negative responses upon exposure towater vapor. Without being bound by any theory or mechanism of action,continuous MCNP film morphology provides the detection of analytesthrough the swelling and dielectric sensing mechanisms, whilediscontinuous MCNP film morphology provides the detection of analytesthrough ionic conduction from the substrate in MCNP-free domains (voids)to the MCNP domains, side-by-side with tunneling conduction within MCNPdomains.

Accordingly, the present invention provides a simple strategy forcontrolling the sensing properties of a sensor comprising conductivemetallic nanoparticies capped with an organic coating, by engineeringtheir morphology through varying the percentage of coverage of thesubstrate. The morphology of the conductive metallic nanoparticlescapped with an organic coating comprises continuous and discontinuousregions comprising voids to allow the detection of water, VOCs, or acombination thereof Each possibility represents a separate embodiment ofthe present invention. In some embodiments, the present inventionprovides the simultaneous detection of water and VOCs, present in thesame sample (e.g. a breath sample).

Within the scope of the present invention are conductive metallicnanoparticles including, but not limited to, Au, Ag, Ni, Co, Pt, Pd, Cu,and Al nanoparticles and combinations thereof. Each possibilityrepresents a separate embodiment of the present invention.

The conductive metallic nanoparticles are coated with an organiccoating. In exemplary embodiments, the organic coating comprises amonolayer of organic molecules. Suitable coating of the conductivemetallic nanoparticles includes, but is not limited to, alkylthiols,e.g., alkylthiols with C₃-C₂₄ chains, arylthiols, alkylarylthiols,alkenyl thiols, alkenyl thiols, cycloalkyl thiols, heterocyclyl thiols,heteroaryl thiols, alkylthiolates, alkenyl thiolates, alkynyl thiolates,cycloalkyl thiolates, heterocyclyl thiolates, heteroaryl thiolates,ω-functionalized alkanethiolates, arenethiolates,(γ-mercaptopropyl)tri-methyloxysilane, dialkyl disulfides andcombinations thereof. Each possibility represents a separate embodimentof the present invention. Exemplary organic coating includes, but is notlimited to, 2-nitro-4-trifluoro-methylbenzenethiol and3-ethoxythiophenol. Each possibility represents a separate embodiment ofthe present invention.

Sensors comprising conductive metallic nanoparticles capped with anorganic coating can be synthesized as is known in the art, for exampleusing the two-phase method (Brust et al., J. Chem. Soc. Chem. Commun.,1994, 7, 801) with some modifications (Hostetler et al., Langmuir 1998,14, 17). In a non-limiting example, AuCl₄ ⁻ is transferred from aqueousHAuCl₄.xH₂O solution to a toluene solution by the phase-transfer reagentTOAB. After isolating the organic phase, excess thiols are added to thesolution. The mole ratio of thiol: HAuCl₄.xH₂O can vary between 1:1 and10:1, depending on the thiol used. This is performed in order to preparemono-disperse solution of gold nanoparticles in an average size of about3-6 nm. Exemplary procedures include, but are not limited to, thiol:Aumole ratios of 10:1 and 1:1 for dodecanethiol and butanethiol-cappedgold nanoparticles, respectively at an average size of about 5 nm. Aftervigorous stirring of the solution, aqueous solution of the reducingagent NaBH₄ in large excess is added. The reaction is constantly stirredat room temperature for at least 3 hours to produce a dark brownsolution of the thiol-capped Au nanoparticles. The resulting solution isfurther subjected to solvent removal in a rotary evaporator followed bymultiple washings using ethanol and toluene. Gold nanoparticles cappedwith e,g. 2-mercaptobenzimidazole can be synthesized by theligand—exchange method from pre-prepared hexanethiol-capped goldnanoparticles. In a typical reaction, excess of thiol, 2-mercaptobenzimidazole, is added to a solution of hexanethiol-capped goldnanoparticles in toluene. The solution is kept under constant stirringfor a few days in order to allow as much ligand conversion as possible.The nanoparticles are purified from free thiol ligands by repeatedextractions. The conductive metallic nanoparticles may have anydesirable geometry including, but not limited to, a cubic, a spherical,and a spheroidal geometry. Each possibility represents a separateembodiment of the present invention.

In some embodiments, the synthesized conductive metallic nanoparticlescapped with an organic coating are assembled (e.g. by a self-assemblyprocess) to produce a film having continuous and discontinuous regions.The term “film”, as used herein, corresponds to a two dimensionalconfiguration of conductive metallic nanoparticles capped with anorganic coating. The thickness of the film of the present inventiontypically ranges from about 1 nm to about 500 nm. The film is typicallydeposited on top of a substrate. Suitable substrates within the scope ofthe present invention include substances which may be rigid or flexible.Within the scope of the preset invention are flexible substrates whichmay also be stretchable. Exemplary substrates include, but are notlimited to, metals, insulators, semiconductors, semimetals, polymers,and combinations thereof. Each possibility represents a separateembodiment of the present invention. In some embodiments, the substrateis a polymer which may be polyimide (e.g. Kapton), polyamide, polyimine(e.g. polyethylenimine), polyester (e.g. polyethylene terephthalate,polyethylene naphthalate), polydimethylsiloxane, polyvinyl chloride(PVC), polystyrene and the like, Each possibility represents a separateembodiment of the present invention. In one embodiment, the substratecomprises silicon dioxide (for example glass or a silicon wafer coatedwith SiO₂). In another embodiment, the substrate comprises indium tinoxide.

According to the principles of the present invention, the conductivemetallic nanoparticles capped with an organic coating are arranged incontinuous and discontinuous regions on the substrate. The discontinuousregions comprise voids which range in sizes from about 10 nm to about500 nm. The density of voids typically ranges between about 3% and about90% of the area of the discontinuous region,

The present invention discloses for the first time that a correlationexists between the morphology of the film of conductive metallicnanoparticles capped with an organic coating and the relative humidity(RH) conditions at time of deposition. Hence, the present inventionprovides a manner of controlling the sizes and percentages of voids as afunction of relative humidity present during the formation process. Thisallows direction of the sensors' characteristics towards the desiredapplications and needs. When needed, the possibility of ionic conductiondue to water can be completely eliminated from the sensor, and whenneeded, it could be achieved, by alternation of the relative humidityconditions present during the process of formation. Accordingly, it ispossible to detect an analyte of interest, wherein the analyte can be apolar or a non-polar VOC or water vapor or both.

Films or assemblies of conductive metallic nanoparticles capped with anorganic coating can be formed on surfaces using a variety of techniqueswell known in the aft. Exemplary techniques include, but are not limitedto,

-   -   i. Random deposition from solution by drop casting, spin        coating, spray coating and other similar techniques.    -   ii. Field—enhanced or molecular-interaction-induced deposition        from solution.    -   iii. Langmuir-Blodgett or Langmuir-Schaefer techniques.    -   iv. Soft lithographic techniques, such as micro-contact printing        (mCP), rep molding, micro-molding in capillaries (HMIC), and        micro-transfer molding (mTM).    -   v. Various combinations of Langmuir-Blodgett or        Langmuir-Schaefer methods with soft lithographic techniques.    -   vi. Printing on solid-state or flexible substrates using an        inject printer designated for printed electronics.

The sensors of the present invention can be configured as any one of thevarious types of electronic devices, including, but not limited to,capacitive sensors, resistive sensors, chemiresistive sensors, impedancesensors, field effect transistor sensors, and the like, or combinationsthereof. Each possibility represents a separate embodiment of thepresent invention. In a non-limiting example, the sensors of the presentinvention are configured as chemiresistive sensors (i.e.chemiresistors).

The present invention further encompasses a system comprising a sensorarray comprising a plurality of sensors (for example between 2 and 20sensors), each sensor comprising conductive metallic nanoparticlescapped with an organic coating having a morphology which comprisescontinuous and discontinuous regions. The system further comprises apattern recognition analyzer.

According to the principles of the present invention, the patternrecognition analyzer receives sensor output signals and analyzes them byat least one pattern recognition algorithm to produce an outputsignature. By comparing an unknown signature with a database of storedor known signatures, volatile organic compounds and, in particularvolatile breath biomarkers can be identified. The analyzer utilizespattern recognition algorithms comprising artificial neural networks,such as multi-layer perception (MLP), generalized regression neuralnetwork (GRNN), fuzzy inference systems (FIS), self-organizing map(SOM), radial bias function (RBF), genetic algorithms (GAS), neuro-fuzzysystems (NTS), adaptive resonance theory (ART) and statistical methodssuch as principal component analysis (PCA), partial least squares (PLS),multiple linear regression (MLR), principal component regression (PCR),discriminant function analysis (DFA) including linear discriminantanalysis (LDA), and cluster analysis including nearest neighbor. Eachpossibility represents a separate embodiment of the present invention.

Additional algorithms suitable for identifying patterns of volatileorganic compounds and optionally quantifying their concentrationinclude, but are not limited to, Fisher linear discriminant analysis(FLDA), soft independent modeling of class analogy (YMCA), K-nearestneighbors (KNN), neural networks, genetic algorithms, and fuzzy logicalgorithms. Each possibility represents a separate embodiment of thepresent invention. In some embodiments, the Fisher linear discriminantanalysis (FLDA) and canonical discriminant analysis (CDA) andcombinations thereof are used to compare the output signature and theavailable data from the database. Other classification techniques mayalso be employed. After analysis is completed, the resulting informationcan be displayed on a display or transmitted to a host computer.

In certain embodiments, the sensors of the present ion comprise one ormore conducting elements. The conducting elements may include a sourceand a drain electrode separated from one another by a source-drain gap.The sensors may further comprise a gate electrode wherein the sensorsignal may be indicative of a certain property of the cappednanoparticles under the influence of a gate voltage. In someembodiments, the sensor signal may be indicative of a capacitanceproperty of the capped nanoparticles. Within the scope of the presentinvention are sensors comprising continuous and discontinuous regions ofconductive metallic nanoparticles capped with an organic coating formedon a substrate comprising a plurality of electrodes (e.g. Auelectrodes). In various embodiments, the distance between adjacentelectrodes which defines the sensing area ranges between about 0.5 μm toabout 3 mm.

The sensor signal may be induced, according to the principles of thepresent invention by a change in any one or more of conductivity,resistance, impedance, capacitance, inductance, or optical properties ofthe sensors upon exposure to VOCs and/or water vapor. Changes in theoptical properties of the sensor(s) can be measured using e.g.,spectroscopic ellipsometry.

The sensor signal is detected by a detection means. Suitable detectionmeans include devices which are susceptible to a change in any one ormore of resistance, conductance, alternating current (AC), frequency,capacitance, impedance, inductance, mobility, electrical potential,optical property and voltage threshold. Each possibility represents aseparate embodiment of the present invention. In additional embodiments,the detection means includes devices which are susceptible to swellingor aggregation of capped nanoparticles as well as devices which aresusceptible to a change in any one or more of optical signal,florescence, chemiluminsence, photophorescence, bending, surfaceacoustic wave, piezoelectricity and the like. Each possibilityrepresents a separate embodiment of the present invention.

In some embodiments, the sensor signal is obtained by swelling of theconductive metallic nanoparticles capped with an organic coating. Inother embodiments, the sensor signal is obtained by ionic conductionfrom the substrate in regions free of conductive metallic nanoparticlescapped with an organic coating and/or by tunneling conduction in regionscomprising conductive metallic nanoparticles capped with an organiccoating. In further embodiments, the ionic conduction is controlled bythe magnitude of an applied voltage.

The present invention further provides a method for detecting VOCs inbreath directly exhaled by a subject or in a sample while concurrentlydetermining the amount of water vapor. The method comprises the exposureof a sensor comprising continuous and discontinuous rations ofconductive metallic nanoparticles capped with an organic coating tobreath or said sample and detecting a signal generated by VOCs and watervapor using a detection means, in particular embodiments, a plurality ofsignals can be detected, each signal being collected at a differentapplied voltage. Accordingly, the amount of water vapor can be deducedfrom the detected signal(s) and consequently subtracted from saidsignal(s) thereby allowing the detection of VOCs in said sample. It isto be understood that the VOCs to be detected comprise non-polar organicmolecules as well as polar organic molecules, and combinations of polarand non-polar organic molecules. Each possibility represents a separateembodiment of the present invention.

In specific embodiments, the sample comprising VOCs and water is abreath sample. The collection of a breath sample, according to theprinciples of the present invention, can be performed in any mannerknown to a person of skill in the art. In exemplary embodiments, thebreath sample may be collected using a breath collector apparatus.Specifically, the breath collector apparatus is designed to collectalveolar breath samples. Exemplary breath collector apparatuses withinthe scope of the present invention include apparatuses approved by theAmerican Thoracic Society/European Respiratory Society (ATS/ERS); Am. J.Respir. Crit. Care Med. 2005, 171, 912). Alveolar breath is usuallycollected from individuals using the off-line method.

The present invention further provides breath analysis which is directedto the diagnosis of various diseases or disorders. Encompassed by thepresent invention is the diagnosis of cancer by the detection of VOCsindicative of cancer while concurrently determining the amount of watervapor, in some embodiments, the cancer to be diagnosed includes, but isnot limited to, lung, brain, ovarian, colon, prostate, kidney, bladder,breast, oral, and skin cancers. Each possibility represents a separateembodiment of the invention.

The present invention further provides the use of the sensor of thepresent invention as a breath sensor, a breath monitor and/or a breathswitch. Each possibility represents a separate embodiment of the presentinvention. The present invention thus provides a method for in-situmonitoring of humidity levels in breath directly exhaled by the subjecton the sensor, suitable for applications such as, but not limited to,diagnosing and treating sleep apnea and other respiratory diseases bydetection of water vapor using a sensor as described herein. The presentinvention further provides a method of preventing sudden infant deathsyndrome (MS) by detection of water vapor present in infant breath usinga sensor as described herein and alerting through generating a humanperceptible signal if infant breath is not detectable. The presentinvention further provides methods of using a sensor as described hereinas an on/off breath switch by detection of water vapor present in breath(single or few subsequent breaths). Accordingly, the sensor detects asignal generated by water vapor present in the breath of a subject andactivates an input device by changing the configuration of an on/offswitch. The on/off breath switch, according to the principles of thepresent invention can be used in instances where touch-buttons arenot-applicable for example by quadriplegic patients, paraplegicpatients, amputees, patients with a spinal cord injury, physicallyimpaired persons, and the like. Each possibility represents a separateembodiment of the present invention. Suitable applications of the breathswitch\button include, but are not limited to, replacement of “Sip andPuff” switch, a pager button, an emergency button, an on/off button foruniversal remote devices (e.g. TV, AC, Video, Stereo, etc.), a lightswitch, a door opening switch optionally combined with a video-intercom,an elevator button, a smartphone controller, and a selection (click)button for an IR guided cursor. Each possibility represents a separateembodiment of the present invention. The sensors of the presentinvention may be incorporated into a device, said device being in a formof a nose clip or a stick-patch, as is known in the art. The sensor ofthe present invention can easily be tailored by controlling thepercentage of continues and discontinuous regions thus enabling thetuning of the switch to trigger at different humidity levels.

As used herein and in the appended claims the singular forms “a”, “an,”and “the” include plural references unless the content clearly dictatesotherwise. Thus, for example, reference to “an organic coating” includesa plurality of such organic coatings and equivalents thereof known tothose skilled in the art, and so forth. It should be noted that the term“and” or the term “or” is generally employed in its sense including“and/or” unless the content clearly dictates otherwise.

The following examples are presented in order to more fully illustratesome embodiments of the invention. They should, in no way be construed,however, as limiting the broad scope of the invention. One skilled inthe art can readily devise many variations and modifications of theprinciples disclosed herein without departing from the scope of theinvention.

EXAMPLES

Materials and Methods

Synthesis, Preparation and Characterization of MCNP Chemiresistors

Gold (III) chloride trihydrate (HAuCl₄.3H₂O), tetraoctylammonium bromide(TOAB), sodium borohydride, 2-nitro-4-trifluoro-methylbenzenethiol(NTMBT), and 3-ethoxythiophenol (ETP) were purchased from Sigma-Aldrich.All reagents were at analytical grade and were used as received.Spherical gold nanoparticles (AuNPs; 3-6 nm in diameter) weresynthesized as described in Peng et al., Nature Nanotechnol. 2009, 4,669; and Brust & Kiely, Colloid. Surf. A 2002, 202, 175. Briefly, asolution of HAuCl₄ was added to a stirred solution of TOAB in toluene.After 10 minutes stirring, the lower aqueous phase was removed. Organicligands and sodium borohydride were subsequently added to the toluenephase. After 3 hours at ice temperature, the lower aqueous phase wasremoved and the toluene phase was subsequently evaporated by rotaryevaporation. After first washing with cold ethanol, the solution waskept at 5° C. for 18 hours until complete immersion was achieved. Thedark brown precipitate was filtered off and washed with ethanol.Chemiresistors (S1-S3) were prepared by drop casting aliquots of thesame MCNP solution under constant relative humidity (RH) atmosphere, inthe range of 10-50% RH, on interdigitated electrodes containing 24 pairsof Au electrodes (5 μm width and 25 μm spacing between adjacentelectrodes) on a silicon wafer with 1000 nm SiO₂ film. In each relativehumidity casting condition, 12 chemiresistors were prepared andcharacterized. The microstructure and morphology of MCNP films werecharacterized by an optical microscope (Nikon Eclipse L150) and by fieldemission scanning electron microscopy (Carl Zeiss Ultra Plus FE-SEM),respectively.

The FE-HRSEM analysis was performed with the aid of two main detectors:Secondary electrons (SE) detector and back scattered electrons (BSE)detector. The SE detector provides high-resolution imaging of timesurface morphology in which inelastic electron scattering caused by theinteraction between the sample's electrons and the incident electronsresults in the emission of low-energy electrons close to the surface ofthe sample. The topography of surface features influences the number ofelectrons that reach the secondary electron detector from any point onthe scanned surface. This local variation in electron intensity createsthe image contrast that reveals the surface morphology. The BSE detectorprovides image contrast as a function of elemental composition as wellas surface topography in which backscattered electrons are produced bythe elastic interactions between the sample and the incident electronbeam. These high-energy electrons can escape from much deeper locationsthan secondary electrons, so surface topography is not as accuratelyresolved as in secondary electron imaging. The production efficacy forbackscattered electrons is proportional to the sample material's meanatomic number, which results in an image contrast which depends on thecomposition. Accordingly, materials with high atomic numbers appearbrighter than materials with low atomic numbers in a backscatteredelectron image.

The morphology of the MCNP films was additionally examined by a tappingmode Atomic Force Microscope (AFM) (Dimension 3100 with Nanoscope lilacontroller, Veeco Instruments Inc.) that is equipped with a 100×100 μm²scanner. Silicon cantilevers with a normal resonance frequency of 160kHz and spring constants of 5 N/m (NSC14/AlBs, MikroMasch, Estonia) wereused. All images were captured with a scan rate of 1-2 Hz and with apixel resolution of 512×512.

In addition, chemiresistors (S4-S9) were prepared b drop castingaliquots of 3-ethoxythiophenol (ETP-AuNP) and NTMBT-AUNP solutions on aflexible foil of polyvinyl chloride (PVC) under constant humidity andtemperature conditions.

Exposure to Analytes

Thirty-six chemiresistors were mounted on a custom PTFE circuit boardwith 40 separated sensor sites (4 sites were left empty). The board wasmounted inside a stainless steel test chamber with a volume of less than300 cm³. Controlled gas concentrations (20-80 ppm of decane or2-ethylhexanol), were produced by a commercial dynamic liquid injectiondilution (DLID) system (Umweittechnik MCZ, Germany). Purified dry air(4.1-7.0% Rti; <0.3 ppm impurities content) from a commercial zero-airsystem (NGA 600-25 MD, Umweittechnik MCZ, Germany) was used as carriergas. The DUD system mixes a constant purified air flow (100±1 cm³/min)with a constant mass flow source of vaporized VOCs. The air/VOC mixtureexiting the DLID system was then diluted with two flow controlleddilution air streams: (i) dry air obtained directly from the zero-airsystem, and (ii) humidified air generated by the system's humidifiermodule. The total VOC concentration at the system output was set bycontrolling the mass flow rate of the vaporized VOC(s) and the totalvolumetric air flow rate. The VOC(s) and RH output were monitored by acommercial photoionization detector (PID; ppbRAE 3000) and by acommercial RH sensor (±2% accuracy), respectively. The sensingexperiments were performed by monitoring the response of the NICNIP andenvironmental sensors (i.e., RH, temperature and pressure sensors) tothe VOCs generated by the DUD system, A Keithley datalogger device(model 2701 DMM) controlled by a custom LabVIEW program was used tosequentially acquire resistance readings from the sensor array andvoltage readings from the environmental sensors. Constant currents inthe range of 0.7-1·10⁴ μA were used for resistance measurements. Atypical exposure cycle involved a 5 minutes vacuum step (<50 mTorr),followed by 5 minutes exposure to the test vapor under stagnantconditions, and ended with another 5 minutes vacuum step. Eachacquisition cycle of the entire sensor array was completed in less thanfour seconds. Each exposure cycle for either water vapor or organicanalytes was typically repeated three times to test for reproducibility.

PH Indicator

Bromothymol blue PH-indicator was dissolved in ethanol and deionizedwater. The indicator was drop-cast on an interdigitated electrode andallowed to dry under a +5 V or 0 V bias between the adjacent electrodesof the interdigitated device.

Example 1 Characterization of MCNP Films

Aliquots of nitro-4-trifluoro-methylbenzenethiol-capped goldnanoparticle (NTMBT-AuNP) solution were drop-cast on interdigitatedelectrodes under various humidity conditions: 16% RH (S1), 31% RH (S2)and 54% RTI (5). Similar baseline resistance, electrical properties andresponses to VOCs and water were obtained for ˜90% of the sensorsproduced in each humidity condition. The varying Relative Humidity (RH)conditions led to the formation of high baseline resistances forrepresentative sensors S1 and S2 (˜18 MΩ) and to the formation ofrelatively low baseline resistance for S3 (˜5 MΩ), FIGS. 1A-1F presentfield emission high resolution scanning electron microscopy (FE-HRSEM)images of the S1-S3 films. As seen in these figures, drop-casting underdifferent RH conditions controlled the morphology of the NTMBT-AuNPfilms. The films deposited under a high humidity level (54% RH) resultedin more uniform layers with less aggregates. In contrast, filmsdeposited under low humidity levels showed less uniform layers with moreaggregates. Particularly, the formation of the S1 and S2 films wasinterrupted by pinholes that are free of NTMBT-AuNPs, thus causing thefilms to be discontinuous (perforated). The S2 and S2 films includedaggregates of NTMBT-AuNPs, with larger aggregates for S1 (10 s-100 s μm)and medium aggregates for S2 (up to 50 μm). Unlike the S1-S2 films, theS3 film was characterized by a uniform and continuous NTMBT-AuNP layerthat has neither pinholes nor aggregates. Similar morphological featurescould be obtained through advanced assembly or deposition techniques(Lee et al., J. Appl. Phys. 2007, 91, 173112; Zhang & Srinivasan, J.Coll. Interf. Sci. 2008, 319, 450).

To further study the morphology nature of the S1-S3 films, combined BSEand SE analyses were performed for two representative samples. The whiteareas which are produced using the BSE detector are attributed to theMCNP film, whereas the dark areas are attributed to the SiO₂ surfacewhich has lower average atomic number as compared to the MCNPs. FIG. 2shows that S3 has a flatter morphology as compared to S1. For S3, imagesthat were obtained from the BSE detector showed only bright areas withalmost no color variation. In other words, the BSE detector showed onlyMCNP film without any evidence of SiO₂ pinholes or areas. For S1, theBSE detector showed some degree of perforation, with the SiO₂ substrateseen as small dark pinholes. Thus, the S1 sensor showed a higher degreeof SiO₂ area compared to the S3 sensor. The HR-SEM confirmed that S1 hasa perforated MCNP film structure, with voids that reach the SiO₂ surface(the dark areas) while the S3 film is continuous with relatively raredefects of perforation, which do not seem to reach the underlying SiO₂surface.

The same S1 and S3 films were scanned using AFM (FIGS. 3A-3B). Thescanned areas were 5×5 μm with an intentionally imposed scratch to allowmeasurement of the MCNP film above the SiO₂ substrate. The measurementof surface roughness of the area enclosed with the broken line rectanglein FIGS. 3A-3B allowed calculation of the films' roughness as follows:(i) Surface area difference that represents the difference between theimage's 3D surface area and its 2D footprint area was calculated as8.83% for S1 and 1.03% for S3; and (ii) Rq, which is theroot-mean-square average of height deviations taken from the mean dataplane (rms) was calculated as 40.4 nm for S1 and 3.74 nm for S3. Thus,the AFM analysis revealed that the average thickness of both S1 and S3films was similar (˜15 nm), while the rms roughness of S1 (40.4 nm) wasmuch higher than that of S3 (3.74 nm).

Example 2 Response to VOCs and Water Vapor

S1-S3 sensors were exposed to various concentrations of decane,2-ethylhexanol (with constant background humidity level of ˜5%) andwater, FIGS. 4A-4C show the responses obtained from triplicates of S1,S2, and S3, prepared in different casting cycles, to different humiditylevels. R_(end) is the resistance at the end of the sensing signal andΔR is the R_(end)-corrected resistance change upon exposure of thesensor to the analyte. The notation Cycle ij describes the sensor'stype, via the numerical digit (i=1, 2 or 3), and the casting cycle, viathe alphabetical capital letters (j=A, B, or C). Evidently, the resultsshow that the variations obtained within a specific batch production(i.e., under a specific RH condition) are much smaller than variationsin responses from different batch productions (i.e., production underdifferent RH conditions). FIGS. 5A-5D show the resistance responses,ΔR/R_(end), of the sensors upon exposure to analytes. As seen in thesefigures, S3 yielded the best signal-to-noise ratio (SNR), compared to S1and S2, upon exposure to VOCs. In contrast, S3 exhibited the lowestresponses, compared to S1 and S2, upon exposure to humidity conditions.For example, the SNR obtained upon exposure to 20 ppm decane was 3.6 forS1, 5.6 for S2, and 19.7 for S3. The responses of S1-S3 upon exposure todecane and 2-ethylhexanol exhibited a reduction in conductivity and anenhancement in the NTMBT-AuNP resistance. The higher the amount of VOCor water concentration, the higher the responses of S3 (see FIGS.5A-5D), indicating sensing by a swelling effect (Joseph et al., J. Phys.Chem. C 2008, 112, 12507; Joseph et al., J. Phys. Chem. B 2003, 107,7406). S1 and S2 showed nonlinear correlation with the RH, with positiveΔR/R_(end) at low RH levels and negative ΔR/R_(end) at high RH levels(Table 1). The results also show that the kinetics of the NTMBT-AuNPsensing response are affected by the RH level. The negative response towater rapidly declined towards the baseline resistance from the momentof exposure. The decline was slower when higher concentrations of waterwere introduced. For example, 66 R/R_(start)=−65% (ΔR is thebaseline-corrected resistance change at the beginning of the signal uponexposure of the sensor to the analyte and R_(start) is resistance at thebeginning of the sensing signal) was obtained immediately after exposureof S2 to 43% RH, but declined to ΔR/R_(end)=−0.80% after 5 minutes.ΔR/R_(start)=−73% was obtained immediately after exposure of S2 to 50.5%RH, but declined to ΔR/R_(end)=−3.2% after 5 minutes, even though thewater vapor was still present in the chamber in both instances. Withoutbeing bound by any theory or mechanism of action, the reduction of theresistance in response to water vapor is associated with the ionizationof condensed water on the SiO₂ substrate and the conduction by thegenerated ions. The fast decline in the response (back to baselineresistance) is attributed to the accumulation of these ions at thevicinity of the oppositely charged electrodes. At the moment of exposureto a humid sample, water condenses on the silicone-oxide and under theapplied voltage, ionization occurs. The amount of produced ions dependson the applied potential and the amount of water. The movement of theproduced ions towards the electrodes spikes a relatively largeelectrical current that rapidly declines once the ions reach theelectrodes. A similar behavior was observed using flexible substrates ona humidity cycle from 5% RH to 94% RH and vice versa (FIGS. 6A-6B).

Another potential source of such ionic currents is the conductionthrough defects and trap states generated in the Si/SiO₂ interface (Ryanet al., App. Phys. Lett. 2011, 99, 223516/1-3; Fischetti, J. Appl. Phys.1984, 56, 575). To test this, S1-S3 films were prepared on circularinterdigitated electrodes (24 pairs of Au electrodes, 5 μm width and 25μm spacing between the adjacent electrodes) on glass substrates ratherthan on Si/SiO₂ substrates. The resulting devices exhibited strongreductions in resistance upon exposure to water and maintained positiveresponses upon exposure to VOCs, similar to the sensing properties ofS1-S3 films on Si/SiO₂ substrates. Additionally, the glass-based S1-S3sensors exhibited similar correlation between the A_(hysteresis) andperforation level compared to the S1-S3 films on Si/SiO₂ substrates.Without being bound by any theory or mechanism of action, thisobservation suggests that the negative responses of S1 and S2 towardwater are not due to conduction through defects and trap statesgenerated in the Si/SiO₂ interface.

TABLE 1 Calculated ΔR/R_(start) and ΔR/R_(end) for S1, S2 and S3 underdifferent humidity levels RH/Sensor S3 S2 S1 response ΔR/R_(end)ΔR/R_(start) ΔR/R_(end) ΔR/R_(start) ΔR/R_(end) ΔR/R_(start) (%) 0.02 00.18 0.1 0.17 0.16 7 0.24 0.75 −0.13 1 −0.9 −2.33 17 0.15 1.25 −0.3−26.9 0.3 −85.7 31 0.25 0.6 −0.8 −65 −6.3 −91.8 43 0.57 0.6 −3.2 −73−40.5 −92.7 51

Example 3 Modeling and Sensing Relative Humidity Conditions

In order to study the response of sensors comprising discontinuousregions to water vapor, the normalized responses, ΔR/R_(base) of sensorsof NTMBT-AuNP and ETP-AuNP vs. humidity (FIGS. 6A-6B) were modeled toproduce a linear equation which represents the change in resistance as afunction of relative humidity (without including relative humiditylevels which produce a drop in resistance due to the accumulation ofions at the vicinity of the oppositely charged electrodes). A blind testanalysis of 20% of the samples was performed using S4 and S5 sensors.The accuracy of the blind test was determined as 86% for both sensors(Table 2). Thus, the sensors of the present invention provide a reliablemeasurement of the relative humidity of a sample by measuring the changein resistance from baseline resistance upon exposure to the sample. Anincrease in accuracy can further be achieved through post-processinganalysis (e.g. increasing the number of line cycles, inducing a delayand oversampling).

In order to determine the response time of the sensors (the time betweentwo adjacent measurements of resistance), the hysteresis of thenormalized responses vs. relative humidity was calculated for a shift of0.2 seconds and 1 minute of the normalized responses. The response timeof sensors of NTMBT-AuNP was determined as 0.2 seconds or lower and theresponse time of sensors of ETP-AuNP was determined as 1 minute (FIGS.7A-7C). Thus, the sensors of the present invention provide fast responseand return times to different levels of relative humidity.

TABLE 2 Statistical analysis of blind test for the modeling ofNTMBT-AuNP versus relative humidity ΔR/R/ Average Sensor 1%/RH Accuracyvariance Equation S4 +0.085 86% ±1.2% RH ΔR/R = 0.085RH − 4.768 S5+0.082 86% ±1.1% RH ΔR/R = 0.082RH − 4.402

Example 4 Hysteresis in Current-Voltage Characteristics

FIGS. 8A-8D show normalized current--voltage (I-V) curves of S1-S3 uponexposure to various RH levels. I-V curves were recorded after reachingsteady state conditions of relative humidity in the exposure chamber.I-V curve measurement can be compared to the initial response uponexposure to relative humidity ΔR/R_(start) since it was the firstapplied voltage on the sensors, thus making the first ionization only atthe time of measurement. As seen in these figures, S1 showed anegligible hysteresis at 20% RH, but large hysteresis upon exposure tohigher RH levels. The normalized hysteresis area, A_(Hysteresis), wascalculated by summing the results of subtractions of the maximumnormalized current value from its minimum value for each given voltage.A_(Hysteresis) was equal for both 40% and 50% RH (A_(Hysteresis) ˜2.2a.u.), suggesting a possible saturation in the response mechanism (Table3). S2 showed no hysteresis upon exposure to 20% RH, but a largehysteresis was present under 40% RH (A_(Hysteresis) ˜0.3 a.u.) and aneven larger hysteresis under 50% RH (A_(hysteresis) ˜1.2 a.u.). For 40%and 50% RH, nonlinear I-V curves were observed for S1 and S2 and Ohmiccurves were observed for S3. The A_(hysyeresis) of S3 was negligibleunder all RH levels applied in the current study (see FIGS. 8C, 8D). I-Vcurves were also measured during exposures to dry air containing eitherdecane or 2-ethylhexanol. No hysteresis was obtained from any of thesensors during the exposure process to these VOCs. For S1 and S2, acorrelation between ΔR/R_(start) and A_(Hysteresis) was found uponexposure to water. R_(start) is the resistance at the beginning of thesensing signal (average of the first 5 points of the exposure process)and ΔR is the baseline-corrected resistance change at the beginning ofthe signal upon exposure of the sensor to an analyte. For S2, theresults showed ΔR/R_(start)=1% at 17.5% RH and A_(Hsteresis) ˜0 (a.u.)at 20% RH; ΔR/R_(start)=−65% at 43% RH and A_(Hysteresis) ˜0.3 (a.u.) at40% RH; and ΔR/R_(start)=−80.5% at 50.5% RH and A_(Hysteresis) ˜1/2(a.u.) at 50% RH. S1 exhibited slightly negative (ΔR/R_(start)=−2.3%)responses upon exposure to 17.5% RH and A_(Hysteresis) ˜0.02 (a.u.) uponexposure to 20% RH. In contrast, S1 exhibited highly negative responsesupon exposure to 43% RH (ΔR/R_(start)=−91.8%) and to 50.5% RH(ΔR/R_(start)=−92.7%), with A_(Hysteresis) ˜2.2 (a.u) for both RHlevels.

TABLE 3 Calculated A_(Hysteresis) for S1, S2 and S3 under 20%, 40%, and50% RH levels S3 S2 S1 RH 0.02 0.03 0.02 20% 0.04 0.3 2.2 40% 0.03 1.22.2 50%

The positive responses of S1 and S2 towards VOCs comply with theswelling mechanism (see FIGS. 5A-5B). In contrast, the correlationbetween ΔR/R_(start) and A_(Hysteresis) upon exposure to humidity rulesout the involvement of a. swelling mechanism in S1 and S2. Additionally,the rapid decline of the responses toward the baseline during exposureto water (see FIG. 5C), rules out sensing response via a tunnelingmechanism (Pavanello et al_(—) J. Phys. Chem. B 2010, 114, 4416; Linkoet al., Nanotechnology 2011, 22, 275610) since the resistance does notreach a new steady-state value. Without being bound by any theory ormechanism of action, the negative response of S1 and S2 towards watervapors is related to the hysteresis effect (Linko et al., Nanotechnology2011, 22, 275610). Accordingly, the sensing process in S1 and S2 mightbe obtained by the conduction of ionic species (Han Ha et al., Chem.Phys. Lett. 2002, 355, 405; Anderson & Parks, J. Phys. Chem. 1968, 72,3662), rather than by tunneling (Wuelag et al., J. Am. Chem. Soc. 2000,122, 11465; Wuelfing & Murray, J. Phys. Chem. B 2002, 106, 3139; Terrillet al., J. Am. Chem. Soc. 1995, 117, 12537).

Example 5 Conductivity in Relative Humidity Conditions

In order to study the effect of RH on the conductivity of the sensors, apH indicator was used. The conductivity is dominated by ionic current asfollows: σ=enμ, where e is the elementary charge, n is the ionicconcentration, and u is the ion mobility. Assuming the mobility is notaffected by RH (in the range of 15-50% RH), the conductivity can becalculated as follows: σ∝n∝exp(−e²/(2ε rRT)), where is the localdielectric constant, R is the gas constant and r is the equilibriumdistance of the charges in neutral species (Anderson & Parks, J. Phys.Chem 968, 72, 3662; Yeh & Tseng, J. Mat. Sci. 1989, 24, 2739).

A Bromothymol Blue indicator (having a blue color in a basic environmentand an orange-red color in an acidic environment) was dissolved in waterand ethanol. The indicator was drop-cast on the same interdigitatedelectrodes used for the MCNP sensors, after which it was allowed to dry.This process was performed twice: with and without applying voltagebetween the two electrodes. Color changes at the various electrodes werecompared after the droplet dried (FIGS. 9A-9C). As seen in FIG. 9A, whenthe indicator dries with no applied voltage, there were no significantcolor variations near or between the electrodes. When the indicator wasdried under applied voltage (FIGS. 9B, 9C) pronounced color variationsnear the two electrodes were observed. In particular, a blue color wasclearly seen near the electrode on which a positive bias was applied,indicating a basic environment. In a similar manner, the orange-redcontour surrounding the negatively biased electrode indicated thatacidic environment was created. Without being bound by any theory ormechanism of action, when applying voltage, the generated OH⁻ speciesmove towards the positive electrode, thus creating a high pH level whichturns the indicator blue. The H₃O⁺ species move towards the negativeelectrode, thus creating low pH conditions which turn the indicatororange-red in the vicinity of the electrode. Accordingly, theseobservations show that water is ionized under the applied voltage, andthat these ions take part in the conduction process during the sensingprocess. When the RH is increased, there is an increase in the localdielectric constant, which enhances the dissociation of water and theionization of the MCNP-free domains of the chemiresistive films, viz.the SiO₂ domains.

For film morphologies containing many pinholes, gaps and topographicalfeatures (MCNP-free domains; FIGS. 1A, 1B), the exposure to water vaporsresults in enhanced adsorption and local condensation of water on theMCNP-free domains, viz. on the SiO₂ surface, leading to ionic currentsand to a temporal reduction in resistance upon exposure (FIG. 5C). Ahigh degree of porosity (perforation) provides a larger area and moreadsorption (SiO₂) sites for water. Without being bound by any theory ormechanism of action, the higher the amounts of adsorbed water (ε=78.5),the higher the local dielectric constant (Paska et al., ACS NANO 2011,5, 5620) and the oxide dissociation to ions (Anderson & Parks, J. Phys.Chem. 1968, 72, 3662). Accordingly, films containing less voids/pinholesexhibit small A_(Hysteresis) and a weak resistance reduction uponexposure to a specific RH level while high RH levels lead to a strongdecline in the resistance and increase in the A_(Hysteresis). Thus, bycontrolling the humidity level and the applied voltage, the ionicconduction of the MCNP sensors can be controlled. FIG. 10 shows theΔR/R_(end) of S2 upon exposure to 5% and 40% REI at different voltagesbetween the electrodes contacting the NTMBT-AuNP film. As seen in thefigure, the different voltages yielded different ΔR/R_(end) uponexposure to 40% RH, while there was no voltage effect on the ΔR/R_(end)upon exposure to 5% RH. These results comply with the presence ofhysteresis only at high humidity levels. Applied rising voltages between1 V to 5 V under 40% RH produced negative responses with risingmagnitude, whereas an applied voltage of 0.9 V produced a positiveresponse. Without being bound by any theory or mechanism of action, thepositive response that was obtained below 0.9 V applied voltageindicates the presence of swelling mechanism and the absence ofionization mechanism. When applying voltages larger than 0.9 V, areduction in resistance occurred (negative response). The higher theapplied voltage, the larger the magnitude of the negative responses.Without being bound by any theory or mechanism of action, thisobservation indicates that the higher the applied voltage, above aspecific voltage threshold, the more significant the role of theionization and ionic current mechanisms. Because the swelling is anadditive mechanism with respect to different VOCs and water (KonivalinaHoick, ACS Appl. Mater. Interf. 2012, 4, 317) the subtraction of theVOCs response from the water response would allow mathematicalseparation of the effect of water from other VOCs in the sample on theMCNP-based chemiresistor. These results provide an additional degree offreedom to deliberately control the MCNP sensing properties in realconfounding environments with high and fluctuating levels of humidity aswell as fouling nonspecific bindings. It is contemplated that the ionicconduction mechanism in MCNP sensors is affected by the humidity leveland the applied voltage.

Hence, the response of monolayer-capped metallic nanoparticle (MCNP)chemiresistors of the present invention to volatile organic compounds(VOCs) and water vapor can be engineered via systematic control of theMCNP film coverage (FIG. 11). The sensor of the present invention canthus be used for breath analysis, breath humidity sensing and breathmonitoring. The fast response and return (recovery) time of the sensorenable its use as a breath switch which provides a detectable change inresistance upon breathing on the sensor (FIG. 12).

While certain embodiments of the invention have been illustrated anddescribed, it will be clear that the invention is not limited to theembodiments described herein. Numerous modifications, changes,variations, substitutions and equivalents will be apparent to thoseskilled in the art without departing from the spirit and scope of thepresent invention as described by the claims, which follow.

1. A sensor for detecting an analyte selected from a volatile organiccompound, water vapor and combinations thereof, the sensor comprisingcontinuous and discontinuous regions of conductive metallicnanoparticles capped with an organic coating, wherein the continuous anddiscontinuous regions differentially detect water vapor and volatileorganic compounds.
 2. The sensor according to claim 1, wherein thecontinuous regions exhibit a positive response upon exposure to volatileorganic compounds and to water vapor, and the discontinuous regionsexhibit a positive response upon exposure to volatile organic compoundsand a negative response upon exposure to water vapor.
 3. The sensoraccording to claim 1, wherein the discontinuous regions comprise voidsranging in size from about 10 nm to about 500 nm.
 4. The sensoraccording to claim 1 configured in a form selected from the groupconsisting of a capacitive sensor, a resistive sensor, a chemiresistivesensor, an impedance sensor, and a field effect transistor sensor. 5.The sensor according to claim 1 which is a chemiresistor comprising afilm comprising continuous and discontinuous regions of conductivemetallic nanoparticles capped with an organic coating formed on asubstrate.
 6. The sensor according to claim 5, wherein the substrate isa rigid substrate or a flexible substrate.
 7. The sensor according toclaim 5, wherein the substrate is selected from the group consisting ofmetals, insulators, semiconductors, semimetals, polymers, andcombinations thereof.
 8. The sensor according to claim 7, wherein thesubstrate a polymer selected from the group consisting of polyimide,polyamide, polyimine, polyester, polydimethylsiloxane, polyvinylchloride, and polystyrene.
 9. The sensor according to claim 1, whereinthe conductive metallic nanoparticles are selected from the groupconsisting of Au, Ag, Ni, Co, Pt, Pd, Cu, and Al nanoparticles andcombinations thereof, and wherein the organic coating forms a monolayeron said conductive metallic nanoparticles.
 10. The sensor according toclaim 1, wherein the organic coating comprises compounds selected fromthe group consisting of alkylthiols, arylthiols, alkylarylthiols,alkylthiolates, ω-functionalized alkanethiolates, arenethiolates,(γ-mercaptopropyl)tri-methyloxysilane, dialkyl disulfides andcombinations and derivatives thereof.
 11. A sensor array comprising aplurality of sensors according to claim
 1. 12. A system comprising thesensor array according to claim 11 and a pattern recognition analyzer,wherein the pattern recognition analyzer receives sensor output signalsand compares them to stored data.
 13. The system according to claim 12,wherein the pattern recognition analyzer comprises at least onealgorithm selected from the group consisting of principal componentanalysis (PCA), artificial neural network algorithms, multi-layerperception (MLP), generalized regression neural network (GRNN), fuzzyinference systems (FIS), self-organizing map (SOM), radial bias function(RBF), genetic algorithms (GAS), neuro-fuzzy systems (NFS), adaptiveresonance theory (ART), partial least squares (PLS), multiple linearregression (MLR), principal component regression (PCR), discriminantfunction analysis (DFA), linear discriminant analysis (LDA), clusteranalysis, and nearest neighbor.
 14. A method for detecting an analyteselected from a volatile organic compound, water vapor and combinationsthereof in the breath of a subject or in a sample, the method comprisingthe steps of: (i) providing a sensor according to claim 1; (ii) exposingthe sensor to the breath of a subject or to a sample comprising saidanalyte; and (iii) detecting a signal generated by said analyte using adetection means.
 15. The method according to claim 14 for detectingvolatile organic compounds (VOCs), while concurrently determining theamount of water vapor, the method further comprising the steps of: (iv)determining the amount of rater vapor from the detected signal; and (v)subtracting the amount of water vapor from the detected signal therebyallowing the detection of VOCs in the breath of a subject or in saidsample.
 16. The method according to claim 15, wherein the signal isdetected at different applied voltages.
 17. The method according toclaim 16 for diagnosing a disease in a subject comprising detectingvolatile organic compounds (VOCs) indicative of said disease, whileconcurrently determining the amount of water vapor.
 18. The methodaccording to claim 14 for monitoring the breathing of a subjectcomprising detecting a signal generated by water vapor present in eachbreath of said subject.
 19. The method according to claim 14 foractivating an input device by a subject comprising detecting a signalgenerated by water vapor present in the breath of said subject, andchanging the configuration of a switch by the detected signal therebyactivating said input device.
 20. A method of manufacturing a sensoraccording to claim 1, the method comprising the step of forming a filmcomprising continuous and discontinuous regions of conductive metallicnanoparticles capped with an organic coating on a substrate in thepresence of water vapor.